Diminished megakaryocyte production represents a serious public health problem in cancer patients subjected to intensive chemotherapy. Understanding the mechanisms of megakaryocyte development will enable pharmacologic enhancement of post-therapy megakaryocyte recovery, preventing significant morbidity and mortality. Understanding the mechanisms of megakaryocyte development will also shed light on the pathogenesis of megakaryocytic leukemias, diverse entities that afflict both children and adults. A major positive influence on megakaryopoiesis consists of protein kinase C (PKC) signaling, and a major negative influence consists of signals resulting from contact with bone marrow stromal cells. We have recently published a novel signal transduction pathway involving PKC isozyme-selective induction of megakaryocytic differentiation: in particular, PKC-epsilon uniquely potentiates activation by the erythro-megakaryocytic transcriptional master regulator, GATA-1, at megakaryocytic promoter elements. Furthermore, we have recently discovered that the blockade of megakaryocytic differentiation by contact with bone marrow stromal cells correlates with the selective, dramatic downregulation of PKC- epsilon expression at the mRNA level. A known target of PKC-epsilon signaling consists of the AP-1 (Fos and Jun) family of transcription factors. In an effort to identify downstream effectors of PKC-epsilon that modulate GATA-1 function, we have discovered that c-Jun, but not other Jun factors, potently represses GATA-1 activation of a megakaryocytic promoter. Therefore, PKC-epsilon signaling, known to upregulate c-Fos, may act to reverse the inhibitory effects of c-Jun on GATA-1. The aims of this project are: 1) Determination of the molecular mechanisms for c-Jun repression of GATA-1 function at a megakaryocytic promoter. 2) Understanding how stromal contact causes selective downregulation of PKC-epsilon and understanding the consequences of this downregulation on AP-1 and GATA-1 functions. Both cell line model systems and primary hematopoietic progenitor cells will be employed for these studies.